Project Summary A major goal of neuroscience research is to understand how the brain integrates novel associations into both short- and long-term memory circuits and extinguishes irrelevant memories. To accomplish this task, many neural circuits involved in learning rely on dopaminergic teaching signals to impart meaning onto a sensory stimulus. Dopamine has been studied extensively in the mammalian brain to understand the basic principles of neuromodulation. However, the complexity of mammalian dopaminergic systems has made it difficult to generate a comprehensive understanding of the role of dopamine at the molecular, synaptic, and neural-circuit level. The simple neural architecture and genetic tractability of Drosophila melanogaster provides an exceptional paradigm to study the role of dopamine in synaptic modulation during learning, and probe the molecular mechanisms involved across both short- and long-term memory. In preliminary data, I demonstrated that dopamine plays an instructive role in shaping synaptic signaling in the mushroom body, an associative olfactory brain center essential to odor learning in Drosophila. Using two-photon imaging, I showed that distinct dopamine signaling cascades are engaged depending on whether dopamine temporally precedes or follows the odor stimulus. This selective recruitment allows for the encoding of meaningful odor memories and the extinction of irrelevant memories through dopamine dependent bidirectional synaptic plasticity. Additionally, my preliminary data suggest that neurons behaviorally implicated in short-term memory exhibit diverse forms of synaptic modulation, while those involved in long-term memory are more resilient to neuromodulatory input. These preliminary data lead to the hypothesis that the neurons involved in short-term memory possess a distinct complement of signaling molecules, endowing them with the capacity for flexible bidirectional plasticity, while those involved in long-term memory have a different molecular milieu that limit dopamine-dependent plasticity. This limited plasticity may maintain synaptic changes and allow for memory maintenance over the enduring time frame of long-term memories. In order to address this specific hypothesis I propose to take advantage of the simple neural architecture and precise genetic toolkit of Drosophila to 1) reveal the molecular mechanisms that underlie the bidirectional synaptic plasticity associated with memory formation and erosion and 2) examine the molecular basis of synaptic plasticity in neuronal populations involved in short- and long- term memory. Results from these two aims will provide a unique opportunity to link the molecular basis of synaptic plasticity with the circuit alterations critical for memory. Given the conservation of dopamine signaling pathways across insects and mammals, this research will provide novel insight into the fundamental properties of dopaminergic reinforcement pathways in learning circuits with important implications to understanding neuromodulation in both healthy and diseased states.